Chair base



FIG. 1 is a perspective view showing the front, top and opposite sides for a chair base which forms my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a hunt elevational view thereof;

FIG. 4 is a bottom plan view thereof;

FIG. 5 is a rear elevational view thereof;

FIG. 6 is a right side elevationalview thereof; and,

FIG. 7 is a eft side elevational view thereof.

The broken in the figures depict portions of the chair base that form no part of my design. 

CLAIM The ornamental design for a chair base, as shown and described. 